One type of a CCD image sensor, in general, comprises a plurality of photodetectors arranged in an array, such as an array of rows and columns, and CCD shift registers between the columns of the photodetectors. The photodetectors in each column are coupled, such as by a transfer gate, to their adjacent shift register so that the charge carriers generated and accumulated in the photodetectors can be selectively transferred to the shift register. The shift registers transfer the charge carriers to the read-out circuit of the imager.
One problem which has arisen in this type of imager is referred to as "blooming." If, during the accumulation period of the imager (i.e., the period when the photodetectors are receiving photons from the image and converting the photons to charge carriers), a photodetector accumulates an excess amount of charge carriers, some of the charge carriers will overflow from the photodetector into the adjacent shift register and/or photodetector. This adversely affects the charge carriers in the shift register being transferred to the read-out circuit causing "blooming."
A conventional technique used for preventing blooming (i.e., an antiblooming technique) uses an overflow drain adjacent the photodetector with the drain being isolated from the photodetector by a potential barrier. The potential barrier between the photodetector and antiblooming drain is lower than the barrier provided by the transfer gate between the photodetector and the CCD shift register during the accumulation period. Thus, if the charge level in the photodetector reaches a sufficient amount to lower the photodetector potential to a level below that of the barrier between the photodetector and the antiblooming drain, additional signal carriers are swept over into the antiblooming drain where they are removed by the drain supply. This prevents excess charge from flowing into the shift register during the integration period and thereby prevents blooming.
The amount of antiblooming protection, X.sub.AB, is defined as the ratio of the exposure level which causes blooming to the saturation exposure level. For a constant integration time and with no, or negligibly small levels of smear, the amount of antiblooming protection is given by: EQU X.sub.AB =exp[q(N.sub.CCD -N.sub.PD)/CV.sub.t ]
where q is the electron charge N.sub.CCD is the charge capacity of the CCD, N.sub.PD is the capacity of the detector element at saturation, C is the detector's capacitance, V.sub.t is the thermal voltage as given by nkT/q, n is the nonideality factor of the antiblooming structure, k is Boltzmann's constant, and T is the absolute temperature of the detector. Hence X.sub.AB is seen to be exponentially dependent on C.
Also, the photoresponse nonlinearity of a photodetector at saturation operating under these conditions is given by: EQU PRNL=[CV.sub.t /RE.sub.SAT ]ln(2)
where R is the detector's responsivity in amps/watt/m.sup.2, and E.sub.SAT is the saturation exposure in joules/m.sup.2. Thus, the photoresponse nonlinearity (PRNL) is linearly dependent on C.
Although simply reducing the area of the photodetector reduces its capacitance and thereby improves antiblooming characteristics, it also results in a reduction in responsivity because of the loss in photoactive area. Therefore, it would be desirable to be able to reduce the storage area of a photodetector so as to reduce its capacitance while maintaining a large photoactive area.